The present invention relates to optical crystals. more particularly, the present invention relates to a periodically-poled structure of ferro-electric crystals.
A periodically-poled structure, which is sometimes called a periodically domain-inverted structure or periodic polarization inverting structure, of ferro-electric crystals, includes periodical inversion of the direction of spontaneous polarization of a certain domain of a crystal. The periodical inversion includes not only repeats of inversion and non-inversion with a single period but also repeats of inversion and non-inversion with inconsistently varying periods to afford objective properties. When a ferro-electric crystal having a periodically-poled structure is used, a quasi-phase matching becomes possible and second harmonic generation and optical parametric oscillation are afforded.
A known method for forming a periodically-poled structure of a ferro-electric crystal is the application of an outside voltage using liquid electrodes. According to this method, an electrolyte solution is used as a liquid electrode, and an electric field is made to act on the crystal via the liquid electrodes, thereby to invert the direction of polarization of the concerned part.
The above-mentioned liquid electrodes are obtained by the use of a liquid metal. This method has a problem in that the removal of a residue of the liquid metal adhered to the surface of the crystal after a polarization inverting process takes a long time. In addition, a metal lead wire placed in contact with a liquid metal to establish an electric contact may be corroded, thus failing to allow a stable polarization inverting process.
Of the ferro-electric crystals, a specific ferro-electric crystal, such as Mg doped LiNbO3 crystal (hereinafter to be also referred to as Mgxe2x80x94LiNbO3 crystal) is drawing attention for its superior resistance to optical damage. However, this ferro-electric crystal easily suffers from disintegration of the crystal (crystal breakage) when forming a periodically-poled structure. Due to an easy leakage of the current through the breakage, therefore, a voltage cannot be applied, making the formation unfeasible.
Even if the problem of disintegration of crystals and the like can be overcome and polarization inversion can be performed, the domain where the direction of polarization has been inverted is still defective. This is because the state of inversion of the +z-plane and of the section from the +z-plane to the xe2x88x92z-plane is irregular and inconsistent, wherein the uniformity of the inversion varies from product to product and the same level of irregularity does not occur again.
It is therefore an object of the present invention to improve the production method of the periodically-poled structure of a ferro-electric crystal and to reduce the incidence of disintegration of the crystal when forming a periodically-poled structure, and further to improve uniformity of polarization inversion and reproducibility of the quality of the resulting products.
According to the present invention, there is now provided a production method of a ferro-electric crystal having a periodically-poled structure, which comprises the steps of:
(a) facing an electrolyte solution, as one liquid electrode, with a domain (region) to be subjected to polarization inversion, which is on one side surface of a substrate comprising a ferro-electric crystal,
(b) facing an electrolyte solution, as the other liquid electrode, with the other surface of the substrate to allow an electric field to act at least on the domain, and
(c) applying a polarization inverting voltage on the substrate via the both liquid electrodes at a temperature of 45xc2x0 C. or above, thereby to invert the direction of polarization.
By setting the temperature to not less than 45xc2x0 C. when applying a polarization inverting voltage via liquid electrodes, the voltage necessary for the inversion can be lowered. As a result, the disintegration of the crystal during the inverting process can be avoided even when an Mg doped LiNbO3 crystal is used, and a desired polarization inverting structure can be formed.
According to the present invention, the temperature during the application of a polarization inverting voltage is more preferably not less than 100xc2x0 C. In the present invention, an electrolyte solution having a boiling point higher than 100xc2x0 C. is used to avoid boiling during processing.